[AMDGPU] Fix DAGTypeLegalizer::SplitInteger for shift amount type

DAGTypeLegalizer::SplitInteger uses default pointer size as shift amount constant type,
which causes less performant ISA in amdgcn---amdgiz target since the default pointer
type is i64 whereas the desired shift amount type is i32.

This patch fixes that by using TLI.getScalarShiftAmountTy in DAGTypeLegalizer::SplitInteger.

The X86 change is necessary since splitting i512 requires shifting amount of 256, which
cannot be held by i8.

Differential Revision: https://reviews.llvm.org/D40148

llvm-svn: 318727